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The UK is about to start trials of wireless charging for moving cars.

What kind of power transfer efficiency is likely to be achievable in such a system, compared to just plugging in the car directly?

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This is not exactly a comprehensive answer, but I found an interesting article on The Institution of Engineering and Technology's website. It is fairly interesting, if not incredibly technically detailed, and I encourage you to check it out for projects to learn more about. I'll talk about the most interesting parts here.

It has many interesting projects listed. Some are for bus routes, and rather than charging along the whole route, they use 'opportunistic charging' to charge at key places (presumably bus stops and traffic lights) to rapidly transfer energy. This is intended to minimize the disruption and possibly cost of construction (imagine having to dig up an entire busy road to add charging coils). This is called "semi-dynamic charging" by Transport Scotland, who are working on one such project.

However, it sounds like in Korea, a more full route system has been operating.

[T]he Korea Advanced Institute of Science and Technology (KAIST) is running two online electric vehicle (OLEV) buses on a 12km continuous charging route in the city of Gumi. It claims 85 per cent maximum efficiency in power transfer.

You're question is focused on the pure efficiency of the charging capacity, but I think also interesting to consider (or possibly implied in the question) is for what cases these technological limitations make actual use a practical exercise. On that note, I will add that this technology may prove of more usefulness on highways, where range is of particular concern in the arena of electric vehicles, leading to projects going to great lengths to provide very close charging stations (25 to 50 miles) like West Coast Green Highway.

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Based upon the link you provided, it doesn't sound like they know for certain what the end technologies will be and how they'll be tuned. So the final answer regarding charging efficiency likely won't be known until the trial is completed.

The Wikipedia article on inductive charging cites a system that claims 86% efficiency for near field inductive charging for vehicles.

This Texas Instruments presentation cites an inductive charging system efficiency in the ballpark of 70%. It also points out that depending upon how a wired charging solution is configured, a better configured inductive charging system may be more efficient for total system efficiency.1 And it's worth noting that the voltage levels used in this study (5-7V) are significantly lower than what vehicle chargers use (120+V).

I believe the biggest challenge in this trial will be the fact that the inductive charging is no longer "near field." That will likely have the greatest impact on overall system efficiency.

1 It's worth calling out the inherent bias from the piece as it's thinly veiled marketing material for newer generation TI chips.

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  • $\begingroup$ I don't see the distinction that TI make in their diagram between inductive and resonant inductive as useful - resonance has been used for at least 40+ years in this role (based on my personal experience of it) and probably since Tesla (the original one). | It's not obvious that they are NOT using NF (near field) technology. With resonance NF can achieve metres of range and here maybe 300mkm seems adequate. Press release here and ... $\endgroup$ Aug 24, 2015 at 16:22
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    $\begingroup$ ... 250 page feasibility study here with lots of detail. $\endgroup$ Aug 24, 2015 at 16:22
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There's a name for "inductive charging system coupled to a moving vehicle": a Linear Induction Motor.

If you tried running the car above a stationary magnetic field, it would just create drag and consume more of the battery's energy than it replenished. It would be like trying to use a battery to power a generator, then using the generator's output to try and charge the same battery.

If you actively modulated the magnetic field so it appeared to be moving forward (relative to the car), you'd be better off at that point forgetting about the battery, disengaging the transmission (putting the car in neutral), and using it AS a linear induction motor to drag the car along.

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  • $\begingroup$ I think it's fair to say that an inductive charging system would rely on some kind of charging signal, like a sine wave, rather than a 'stationary magnetic field', which may or may not be causing excessive drag. Your idea to just use a linear induction motor is a good one though, I think! $\endgroup$
    – Dan
    Aug 24, 2015 at 20:06

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